1. Field of industrial application
This invention relates, for example, to a hydraulic clutch applicable to a transmission for a construction machine, an industrial machine or a passenger car.
2. Prior Art
FIG. 1a shows a conventional hydraulic clutch which includes an output shaft 1 having a cylinder 2 formed integrally therewith. A hydraulic piston 3 is disposed in the cylinder 2, and a constant clearance l is provided between the piston 3 and a friction member 4 (shown only partly). Further, a return spring 7 (coil spring) is compressively interposed between the piston 3 and a stopper member 6 on the output shaft 1.
In such a hydraulic clutch, the constant clearance l is provided between the friction member 4 and the piston 3 in order not to transmit torque at a time of disengagement. The torque is not transmitted until the piston 3 travels through this clearance l and applies a load to the friction member 4. The time from oil pressure being sent to the piston to a load being applied to the friction material 4 is designated as "time lag". It is desired to shorten this time lag as far as possible. Further, from a standpoint of drive feeling, a characteristic by which torque is transmitted slightly at the beginning of engagement and gradually increasing thereafter is generally recognized as perferable during the period from when the friction member 4 commences transmission of torque until it completes the same.
In order to improve the time lag and the torque transmission characteristics, various means have been contrived. FIG. 1b is an example of them, in which a coned disc spring 5 for lessening a shock at a time of engagement is interposed between the piston 3 and the friction material 4. However, this construction also requires the same clearance l as the hydraulic clutch of FIG. 1a, and in order to shorten the time lag, the piston 3 must be pushed by a higher hydraulic pressure because the piston 3 has inertia. When engaging the clutch, this excessive hydraulic pressure causes a shock which worsens the torque transmission characteristics. Further, in order to gradually increase the load, contrivances such as using plural coned disc springs etc. become necessary and thus cause a cost increase. Furthermore, the above-mentioned hydraulic clutch requires the return spring 7 in addition to the coned disc spring 5, thereby including a problem of an increase in the number of components.